Though drug addiction is one of the most prevalent and devastating diseases, still, relatively little is known about the molecular mechanisms that govern the long-term changes that occur in the brain that lead to addiction. It has been reported for some time that circadian rhythms are important in determining drug sensitivity and sensitization. More recently, studies have begun to emerge showing a more direct role for certain genes that make up the circadian clock in regulating not only drug-induced locomotor activity, but also the rewarding properties of drugs of abuse. This comprehensive study will examine the role of two of the primary transcription factors that make up the circadian clock, Clock and NPAS2, in regulating addiction-related behaviors. First, we will determine if cocaine (like methamphetamine) regulates the expresssion of these genes in specific regions of the brain known to be involved in drug reward. For behavioral and molecular studies, we have acquired mice that have functional knock-outs of Clock and NPAS2 and find that both mutations affect the rewarding properties of cocaine. These mice, along with mice that overexpress or knock-down these genes in specific brain regions through viral-mediated gene transfer, will be tested in a variety of behavioral paradigms to determine if mutation or overexpression of these genes leads to differences in addiction-related behaviors including self-administration, drug seeking and relapse. Finally, we will examine the molecular functions of Clock and NPAS2 to determine how they regulate two transcriptional target genes, mPerl and mPer2, that are involved in regulating drug-induced behaviors. We will determine the DMA binding at the promoters of these genes after drug exposure using gel shift analysis and ChIP, and determine if either Clock or NPAS2 are necessary to induce the Per genes after drug treatment. These studies will greatly enhance our understanding of the molecular changes that occur in the brain in response to drugs of abuse and how the circadian clock modulates drug addiction.